1. Field of the Invention
The present invention relates to light source units and light irradiation units, more particularly to a light source unit that comprises a wavelength converter, which converts light having a single wavelength emitted from a laser light source into light having a different wavelength, and a light irradiation unit that comprises the light source unit.
2. Description of the Related Art
Conventionally, light irradiation units have been used for fine structure inspection of objects, fine processing of objects, and for vision correction treatment. For example, in a lithographic process for manufacturing semiconductor devices or the like, in order to transfer a pattern formed on a mask or a reticle (hereinafter generally referred to as a “reticle”) onto a substrate such as a wafer on which a resist or the like is coated or a glass plate (hereinafter appropriately referred to as a “substrate” or a “wafer”) via a projection optical system, exposure apparatuses are used, which is a type of a light irradiation unit. As such an exposure apparatus, a static exposure type projection exposure apparatus that employs a step-and-repeat method, or a scanning exposure type projection exposure apparatus that employs a step-and-scan method is mainly used. In addition, for vision correction, a laser treatment system, which is also a type of light irradiation unit, is used to perform ablation of a corneal layer (PRK: Photorefractive Keratectomy) or ablation of inner cornea (LASIK: Laser Intrastromal Karatomileusis) for treatment of nearsightedness, astigmatism, or the like.
Many light sources that generate light having a short wavelength have been developed for such light irradiation units. The direction of development of such light sources with short wavelengths can be mainly divided into the following two groups. One is the development of an excimer laser light source whose laser oscillation wavelength itself is short, and the other one is the development of a short wavelength light source that makes use of harmonic generation of infrared or visible light laser.
Of such development, along the direction of the former group, a light source unit that uses a KrF excimer laser (wavelength: 248 nm) has been developed, and at present, alight source unit that uses an ArF excimer laser (wavelength: 193 nm) or the like is being developed as a light source having shorter wavelength. However, such excimer lasers have their disadvantages as light source units, such as their large size, and their complicated maintenance operation and high running cost due to hazardous fluorine gas used.
Therefore, a method of shortening the wavelength along the direction of the latter group is gathering attention, which is a method of converting long wavelength light (such as infrared light or visible light) into ultraviolet light with a shorter wavelength by using a nonlinear optical effect of a nonlinear optical crystal. As a light source that uses such a method, one disclosed in International Publication WO99/46835 pamphlet and the corresponding U.S. Pat. No. 6,590,698, (hereinafter simply referred to as a “conventional example”), for example, are available.
In the wavelength shortening method using nonlinear optical crystals as in the method described above, the generating efficiency of short wavelength light depends on the generating efficiency of the nonlinear optical effect of the nonlinear optical crystal. The higher the brightness (includes the meaning of a “peak power”) of incident light whose wavelength is to be converted is, the higher the generating efficiency of its nonlinear optical efficiency becomes. Therefore, in order to obtain ultraviolet light with good efficiency, high brightness infrared light or visible light has to be incident on the nonlinear optical crystal. So, in the above conventional example, infrared light or visible light of a single wavelength generated by a semiconductor laser or the like is amplified by an optical fiber amplifier that has an amplifying optical fiber in which a rare earth element such as erbium (Er) is doped, and the light is made to be incident on the nonlinear optical crystal. In such an optical fiber amplifier, an optical amplifying function is given to the amplifying optical fiber by supplying exciting light to the amplifying optical fiber to excite the doped rare earth element and to form population inversion regarding the energy level of outer shell electrons of the rare earth element.
When the light having a long wavelength (infrared light, visible light) is converted into ultraviolet light having a shorter wavelength as in the art of the conventional example described above, wavelength conversion is performed by generating a secondary harmonic or a sum-frequency of the incident light, using the nonlinear optical effect of the nonlinear optical crystal. However, in such a case, the following disadvantages occurred.    (1) When obtaining an m-fold wave (m: natural number) of a fundamental wave, in the case an (m-1)-fold wave is already obtained, a sum-frequency generation of the (m-1)-fold wave and the fundamental wave is performed in order to obtain light having a desired wavelength. Normally, as the fundamental wave used for generating the sum-frequency, a fundamental wave is used that has passed through the nonlinear optical crystal in the previous step without being converted into a harmonic. However, the output level of the fundamental wave having passed through the nonlinear optical crystal in the previous step and reached a nonlinear optical crystal in the final step has attenuated significantly, and in such a case, sufficient wavelength conversion efficiency cannot be obtained.    (2) The wavelength that can be converted is limited to a wavelength of the m-fold wave of the fundamental wave, depending on the wavelength of the fundamental wave used for wavelength conversion.